Lecture 10 High-Energy Astrophysics: Synopsis

M.Phys. & M.Math.Phys.

High-Energy Astrophysics

High-Energy Astrophysics MT 2016 Lecture 10 High-Energy Astrophysics: Synopsis

1) Supernova blast waves; shocks. 2) Acceleration of particles to ultra-relativistic energies. 3) Synchrotron emission; total power and spectrum 4) Synchrotron self-absorption, population ageing, radio galaxies. 5) Accretion discs: structures and luminosities. 6) Accretion discs: spectra, evidence for black holes. 7) Relativistic jets; relativistic projection effects; Doppler boosting. 8) Cosmic evolution of AGN; high-energy background radiation and cosmic accretion history of black holes. 9) Bremsstrahlung; inverse-Compton scattering; clusters of galaxies; Sunyaev- Zel’dovich effect. 10) Cosmic rays and very-high-energy gamma rays; Cherenkov telescopes. Today’s lecture

• Cosmic rays.

• Interactions in the atmosphere; air showers.

• Cherenkov radiation.

• Sources of cosmic rays. Cosmic rays

The earth is hit by elementary particles and atomic nuclei of very large energies. Most of them are protons (hydrogen nuclei) but all nuclei up to uranium have been detected (although anything heavier than nickel is very, very rare).

Other energetic particles are mainly electrons and positrons, as well as gamma-rays and neutrinos. Cosmic rays discovered in 1912 by Hess

Cosmic rays were discovered in 1912 by Victor Hess, when he found that an electroscope discharged more rapidly as he ascended in a balloon. He attributed this to a source of radiation entering the atmosphere from above, and in 1936 was awarded the Nobel prize for his discovery.

Air showers

• The incident particles interact with nuclei in the atmosphere, usually several ten of kilometres up, and produce a cascade or shower of secondary particles. • Most of the new particles are pions (+,-,neutral). Neutral pions very quickly decay, usually into two gamma-rays. Charged pions also decay but after a longer time. Therefore, some of the pions may collide with yet another nucleus of the air before decaying, which would be into a muon and a neutrino. The fragments of the incoming nucleus also interact again, also producing new particles. • The gamma rays from the neutral pions may also create e+ /e− pairs and these pairs in turn may produce further gamma rays by inverse-Compton scattering and bremsstrahlung.

• Air showers are also created by very high energy gamma rays coming directly from astrophysical objects. Hadronic Air Shower (primary is a proton or other nucleus) 2.1 Air Showers 17

Figure 2.2: A primary hadron generates a hadronic cascade. The length of the arrows does not correspond to the lifetime of the particle.

The lateral development of electromagnetic showers is determined by elastic multiple • coulomb scattering of electrons. The mean scattering angle of electrons with energies close to the critical energy Ec is rather small and hence the lateral spread of the electromagnetic shower is small. The secondary particles participating in the strong component of the hadronic shower receive a higher transverse momentum in their production. According to Heisenberg’s uncertainty principle an extended target adds to the interaction a typical transverse momentum that corresponds to its size. The size of 1 fm corresponds to 200 MeV, which is the mean transverse momentum of the particles in strong interactions. Therefore the lateral extension of the hadronic shower is much larger. The diﬀerence in the lateral extension of electromagnetic and hadronic showers becomes apparent in Figure 2.3 that shows the particle trajectories for simulated photon- and proton-induced air showers. complex multiparticle processes are involved in the development of the hadronic • shower in contrast to mainly three-particle processes like bremsstrahlung and pair production in the electromagnetic shower. Therefore the hadronic shower is less regular, has larger ﬂuctuations, and contains electromagnetic subshowers that are created by neutral pion decays (see Figure 2.3).

2.1.2 The Emission of Cherenkov Light Charged relativistic particles travelling at a velocity v exceeding the phase velocity of light c in that medium emits Cherenkov light. In the classical picture, the charged particle polarises the surrounding medium and induces constructive interference of electromagnetic Electromagnetic Air Shower (primary is a gamma-ray)

2.1 Air Showers 15

E0/2

E0/4

E0/8

Figure 2.1: Illustration of the shower model according to Bethe and Heitler. Bremsstrahlung and pair production are considered, the radiation lengths for both processes are set equal.

if the average energy falls below the “critical energy” Ec, where the loss of energy per unit length by bremsstrahlung falls below the loss of energy per unit length by ionisation. In this case no new particles are created but the particles lose energy mainly by ionisation and the shower dies out. A simple shower model was ﬁrst introduced by Bethe & Heitler (1954) and relies on very basic assumptions. The main properties of an air shower can however be understood in this model. Only bremsstrahlung and pair production are considered. Energy loss by ionisation is neglected which is a valid approximation for high energetic particles. Both the radiation length for bremsstrahlung and pair production are set to X0, neglecting the factor 9/7 that relates them. In this model, a primary γ-ray enters the atmosphere and generates within one radiation length an electron-positron pair via pair production. Hereby its energy E0 is assumed to be equally divided between the two particles. By bremsstrahlung both the electron and positron generate in turn after exactly one radiation length a photon containing half of their energy (see Figure 2.1). After m radiation lengths X0 the cascade consists of:

m m N(m) = 2 Particles having energy E(m) = E 2− (2.1) 0 · The depth mmax of the maximum of the shower in the atmosphere in units of the radiation length is given by:

mmax ! ln (E0/Ec) E(m ) = E 2− = E m = (2.2) max 0 · c → max ln 2 at this depth mmax, the shower consists of Nmax particles

mmax ln 2 E0 Nmax = e · = (2.3) Ec

Cherenkov Radiation Cherenkov Radiation: Emission Geometry

• A particle is moving at a speed bc through a medium of refractive index n, such that the speed of the particle is greater than the speed of light in the medium. • In time t the particle travels a distance bct but the light emitted travels a distance (c/n)t. • The opening angle of the radiation cone is thus given by:

cos q = 1/(bn) Cerenkov radiation cont’d.

For very high energy particles traveling through the Earth’s atmosphere, the light is emitted on a narrow cone around the direction of the particle, and is strongly concentrated in the blue/UV part of the spectrum (photon energies of around one-half to one eV)

The refractive index of air is 1.0003 at sea level and decreases with altitude, as the density of air decreases.

The opening angle is thus a function of the density of the air and of the height of emission. It increases downwards but is always less than about 1.4 degrees.

From each part of the particle track the Cherenkov light arrives on a ring on the ground.

The Cherenkov light pulse as observed on the ground is extremely short – typically a few nanoseconds – because the photons follow the emitting particles so closely. Simulations of ideal Cherenkov shower patterns

This is the light pool on the ground viewed from directly above. Typically the size is a few hundred metres across.

Left: proton, energy 1 TeV. Centre: Fe nucleus, energy 5 TeV. Right: γ-ray, energy 300GeV.

Cerenkov telescopes Cherenkov Telescope Array - artist’s impression of completed array in Chile Auger Observatory (Mendoza, Argentina). Detecting Cherenkov light with an array of water-filled detectors, sensitive to energies > 1021 eV Cosmic rays: Energy Spectrum Cosmic rays: observed properties

• Mostly nuclei; a couple of percent e+ /e−

• About 80% of the nuclei are protons; 15% He nuclei.

• Energy distribution has power-law form similar to that inferred for shock acceleration. There are two breaks in the spectrum: at ∼ 1016 eV and at ∼ 1019 eV. Origins of cosmic rays

• Correspondence of power-law energy distribution means that cosmic rays could in principle have galactic (supernovae) or extragalactic (AGN) origin.

• Some elements—e.g. Li, Be, B—are overabundant compared with the element abundances in the Solar System. This is believed to be because of spallation in interstellar space of C,O etc. The heavy nuclei break up by collision with protons in the interstellar medium.

• This evidence for spallation implies that the heavy nuclei are conﬁned to the Galaxy. Spallation Conﬁnement for protons/nuclei

• From the overabundance of elements which are the products of spallation, we can infer the column density of material they have passed through on their way to us: roughly 50 kg m-2.

• The density of the ISM in the Milky way is typically 106 particles m−3

• So if the cosmic rays propagate at ≈ c, they have a typical lifetime in the galaxy of 106 years.

• But the scale size of the Milky Way is only a few kpc: therefore the heavy cosmic ray particles must be largely conﬁned in the Milky Way.

It is now generally accepted that most of the cosmic ray particles below the “knee” at 1015 eV are produced within in the Milky Way in SN shocks. Ultra-high energy cosmic rays

However there is still some argument as to the origin of the highest-energy cosmic rays.

The arguments in favour of extragalactic origins are (in very broad terms!)

• The ultra-high energy cosmic rays seem to be distributed around the whole on the sky; they are not conﬁned to the plane of the galaxy.

• For energies > 1015 eV the gyro-radius of a proton in the equipartition ﬁeld of a supernova remnant becomes comparable to the size of the remnant; hence these energies can’t be obtained by SN shock acceleration in the Milky Way. The energies could however be obtained in stronger, AGN, shocks. Ultra-high energy cosmic rays cont’d

The arguments against the highest-energy cosmic rays originating in distant AGN are:

• Protons of sufﬁciently high energy can interact with CMBR photons to produce pions, in reactions such as p + γ → n + π+ .

• The threshold proton energy for this reaction is ≈ 5 × 1019 eV (when scattering against CMBR photons of typical energy ∼ 10−4 eV).

• Using the known density of CMBR photons in the local universe, we should therefore not expect cosmic rays of energy > 1019 eV to propagate farther than 30 Mpc.

• But this distance is not far enough to get us to the large population of (isotropically distributed) AGNs. The number of detected events is still small. Can they be associated with nearby extragalactic objects? Ultra-high energy cosmic rays cont’d

So we need sources of ultra-high energy cosmic rays which:

• Can accelerate the most energetic particles known.

• Do not suffer from the pion-production cutoff, so must be in the nearby (~100 Mpc) universe

• AGN?

• Gamma-ray bursts?

• Exotic fundamental physics? Cosmic Rays: Components Nearby AGN (red) overlaid with detections of cosmic ray particles with energy > 57 EeV